Magnetic recording media usually comprise magnetic pigments in a binder, typically a polymeric resin. Such media often use advanced magnetic pigments which possess a high saturation magnetization (&gt;140 emu/g) and coercivity (&gt;2000 Oe). Furthermore, to increase the storage capacity of recording media per unit area, the length of acicular particles or diameter of hexagonal particles has been reduced, increasing the surface area of pigments to in excess of 45 m.sup.2 /g. The high surface area can render these pigments difficult to disperse.
Magnetic properties can also be improved by increasing the amount of magnetic pigment in the media, by, for example, reducing the amount of non-magnetic material, such as binder, lubricants, dispersing agents, etc. However, such improvement must not sacrifice too much of the physical or mechanical properties of the media. For example, the media must contain sufficient amounts of binder to maintain mechanical strength and durability. Similarly, if the amount of dispersing agent is reduced, care must be taken to provide equivalent or better dispersion properties for the increasingly difficult to disperse magnetic particles.
The binder systems in magnetic media must maintain the extremely small magnetic particles in a fixed position; this permits the particles to be subsequently magnetized and demagnetized, and thus impart a strong, modulated magnetic signal over long exposure to a wide range of environmental and mechanical stresses. Therefore, the binder system must be capable of achieving high tensile strength, flexibility and toughness with high modulus. Preferably, the binder utilizes chemistry which promotes wetting and dispersion of pigment(s).
Traditionally, good pigment wetting has been achieved by selecting suitable dispersing agents, which interact strongly with the pigment surface, and to provide stability to the magnetic particles once dispersed. The move towards higher pigment loading (in excess of 75% by weight of the magnetic layer), and higher pigment surface area, has dictated the need to use higher levels of dispersant at the expense of lower relative levels of binder. In order to overcome the conflict between high pigment loading and increased dispersant content, self-wetting polymers, i.e., multi-functional binders containing "dispersing" groups, have been developed that combine the roles of forming a durable binder matrix and pigment dispersion. These polymers reduce the amount of low molecular weight dispersing agent needed to disperse the particles. Particular polymers containing polar moieties, such as carboxylic acids, sulphonic acids and phosphoric acids, quaternary ammonium groups etc., have been introduced to improve the binders' affinity towards the pigment surface. Good cohesion between pigment and binder, together with good binder mechanical properties ensures that the resultant media exhibits an excellent blend of mechanical and magnetic properties.
Nevertheless, the advances in magnetic pigment technology demand further wetting and modification of the pigment surface to assist the self-wetting polymers to achieve more uniform dispersion of the magnetic pigment. The use of self-wetting binders alone may not produce the desired dispersion properties with the advanced metal particle pigment.
Previously, pigment surface treatment has involved the use of coupling agents, such as, silanes, titanates; or chelating agents, such as, aluminum metal complexes; often during the pigment manufacturing stage to produce pre-treated pigment.
Pigment surface treatments are disclosed in U.S. Pat. No. 5,064,687, U.S. Pat. No. 5,176,955 and U.S. Pat. No. 5,318,838 which describe magnetic recording media having ferromagnetic powder which is treated with an aromatic phosphoric, phosphonic or sulfonic acid and dispersed in a vinyl chloride copolymer. A preferred exemplified surface modifier is phenylphosphonic acid.
It is desired to improve the dispersion properties of magnetic pigments.